Strip-type directional coupler having elongated aperture in ground plane opposite coupling region

ABSTRACT

A compact, microstrip directional coupler is described wherein two narrow strip-like conductors are closely spaced to each other so as to provide a signal coupling region therebetween. The two narrow conductors are spaced from a ground planar conductor by a substrate of dielectric material having a relatively high dielectric constant compared to the adjacent medium wherein the odd mode phase velocity of a signal propagating along the coupler is unequal to the even mode phase velocity thereof. An elongated aperture is located in the ground planar conductor in the coupling region to alter the odd mode phase velocity so that it is equal to the even mode phase velocity.

United States Patent Napoli 1 51 Apr.25, 1972 [54]; STRIP-TYPEDIRECTIONAL COUPLER HAVING ELONGATED APERTURE IN GROUND PLANE OPPOSITECOUPLING Primary Examiner-Paul L. Gensler Attorney-Edward J. Norton [57]ABSTRACT A compact, microstrip directional coupler is described whereintwo narrow strip-like conductors are closely spaced to each other so asto provide a signal coupling region therebetween. The two narrowconductors are spaced from a ground planar conductor by a substrate ofdielectric material having a relatively high dielectric constantcompared to the adjacent medium wherein the odd mode phase velocity of asignal propagating along the coupler is unequal to the even mode phasevelocity thereof. An elongated aperture is located in the ground planarconductor in the coupling region to alter the odd mode phase velocity sothat it is equal to the even mode phase velocity.

2 Claims, 6 Drawing Figures STRIP-TYPE DIRECTIONAL COUPLER HAVINGELONGATED APERTURE IN GROUND PLANE OPPOSITE COUPLING REGION Thisinvention relates to directional couplers and more particularly to anasymmetrical strip transmission line directional coupler.

With the coming of miniaturization to microwave systems, there has beenthe development and use of a form of asymmetrical strip transmissionline wherein a narrow strip-like conductor is spaced by a highdielectric constant material from a single broad planar conductor. Thisform of strip transmission line is often referred to as "microstrip"transmission line. This microstrip" transmission line can be fabricatedby the use of low cost printed circuit techniques and when usingrelatively high dielectric constant material is very compact in size.

Microstrip directional couplers wherein two such narrow strip-likeconductors are closely spaced from a common ground planar'conductor andare closely spaced from each other along a given coupling region areknown. Microstrip directional couplers using relatively high dielectricconstant material are however relatively recent and have been associtedwith miniaturized microwave devices and systems.

When electromagnetic signals are applied to one of the narrow strip-likeconductors of such a directional coupler, there is associated at thecoupling region odd and even mode excitation conditions. In a microstripcoupler, a larger portion of the electric field of the signalspropagating in the odd mode is located outside of the relatively highdielectric substrate in the ambient air than for the signals propagatingin the even mode. Because of this propagation difference for signals inthe even and odd mode and because of the dielectric difference betweenthe substrate and the ambient air, the phase velocity of the signaltraveling in the odd mode is different from that in the even mode. Whenthe dielectric constant of the substrate material is substantiallygreater than that of the ambient air, such as above five, in order toreduce the size of the coupler and confine the fields of signals appliedthereto, there is a marked difference between the odd and evenmode phasevelocity. This marked difference between the odd and even mode phasevelocity causes substantial deterioration in the signal directivity.

It is an object of this invention to provide an improved highlydirective microstrip directional coupler having a substrate ofrelatively high dielectric material.

Briefly this object is accomplished by a directional coupler whereinfirst and second narrow strip-like conductors are spaced from a groundplanar conductor. The first and second narrow strip-like conductors areclosely spaced to each other andto the ground planar conductor so as toprovide an electromagnetic coupling region therebetween. In response toelectromagnetic signals applied thereto, these signals propagate in theodd and even mode along the coupling region of the coupler. The spacingbetween the narrow strip-like conductors and the ground planar conductoris provided by a substrate having a dielectric constant of at least sixcausing the inherent phase velocity of signals propagating in the oddmode to be substantially different from the signals in the even mode.The odd mode only is perturbed sufficiently to an extent such that theodd mode phase velocity is equal to the even mode phase velocity.

The above-mentioned invention will be better understood by reference tothe following description taken in conjunction with the accompanyingdrawings in which:

FIG. 1 is a top plan view of a prior art directional coupler,

FIG. 2 is a cross sectional view of FIG. 1 illustrating odd modepropagation,

FIG. 3 is a cross sectional view of FIG. 1 illustrating even modepropagation,

FIG. 4 is a top plan view of a directional coupler in accordance withthe principles of the present invention,

FIG. 5 is a cross sectional view of the coupler of FIG. 4 taken alongaxis 5-5, and

FIG. 6 is a plot of coupling versus frequency for a directional couplerof the type following the principles of the present invention.

Referring to FIGS. 1 and 2, there is illustrated a perspective view of aprior art edge coupled microstrip directional coupler 10. Two narrowstrip-like conductors 13 and 15 are fixed to one surface of a dielectricsubstrate 11. A closely spaced ground planar conductor 12 is located onthe opposite surface of the substrate 11. The conductors 13 and 15 areclosely spaced to each other over a coupling region 17 which is usuallyabout a quarter of a wavelength along at the center operating frequencyof the coupler. Electromagnetic signals coupled across conductors l3 and12 at one end 16 propagate along conductor 13 toward the opposite end18. At the com,- mon coupling region 17 where conductors '13 and 15 areclosely spaced to each other relative to the ground planar conductor 12,the electromagnetic signal energy propagating along this coupling region17 propagates in both odd and even mode. i

Referring to the odd mode condition illustrated in FIG. 2. the voltageat one of the conductors 13 is positive with respect to the groundconductor 12, and the'voltage at the other narrow conductor 15 isnegative with respect to ground conductor 12. Also the current in theodd mode as indicated by ar- I rows 19 and 21 is in one direction (intothe paper of FIG. 2) along conductor 13 and in an opposite direction(out of the paper of FIG. 2) along conductor 15. In addition, asindicated by arrow 22, there is a current that is transverse to thedirection of propagation along the ground planar conductor 12. As can beseen, referring to FIG. 2, the electric field of the electromagneticsignal energy, as indicated by the dashed arrows, is between the narrowconductors 13 and 15 with a sub stantial portion thereof being locatedoutside of the substrate 1 1 Referring to FIG. 3, there is illustratedthe even mode condition wherein the voltages on the conductors 13 and 15are both positive with respect to the ground planar conductor 12. Thecurrents travel along the conductors 13 and 15 for the even modecondition, as indicated by arrows 24 and 25 into the paper and arrow 26out of the paper, in the same direction, and there is no transversecurrent associated with the even mode condition. Also, the electricfield as indicated by the dashed arrows is mainly within the substrate11.

If the phase velocity for both the odd and even modes is equal, then theelectromagnetic signal energy propagating along conductor 13 from end 16toward end 18 is coupled to a given terminal end 21 of conductor 15. Theratio of the coupled energy to one terminal end 21 'of conductor 15 withrespect to the other end 23 of conductor 15 is the directivity. Highdirectivity is dependent on the phase velocities of the even and oddmode being equal. If the even and odd mode phase velocities aresubstantially different, the directivity deteriorates substantially andbecomes lower as the velocity difference increases.

Referring to FIGS. 2 and 3, it can be noted that a larger portion of theelectric field is outside the dielectric substrate 11 into the ambientair in the odd mode than for the even mode. When the dielectric constantof the substrate 11 becomes higher than about 5 there is a substantialdifference between the odd and even mode phase velocities because of themarked difference in dielectric material medium the signal ispropagating. Consequently, because of this difference in the even andodd mode phase velocities, the directivity of such couplers is low anddeteriorates even further with higher dielectric materials.

Referring to FIGS. 4 and 5 there is illustrated a coupler 30 comprisinga dielectric substrate 31 having a ground planar conductor 32 on onesurface and the narrow strip-like conductors 33 and 35 closely spaced atcoupling region 37 to each other on the opposite surface of saidsubstrate 31. The narrow the narrow conductors 33 and 35. Referring tothe cross sectional view of FIG. 5, this aperture or slot 38 is placedin the ground planar conductor 32 such as to be centered below thecoupling region 37. Referring to FIGS. 2 and 3, it is noted that in theeven mode excitation, there is no component of current at the groundplane transverse to the direction of propagation of the signals, butthere is a component of current for the odd mode excitation that istransverse to the direction of propagation of the signals. By theplacement of an elongated aperture or slot 38 in the ground conductor 32at the center line, the transverse odd mode current as indicated byarrow 22 of FIG. 2 is interrupted, but the even mode current is not.

When the odd mode current is interrupted by an elongated aperture, asillustrated in FIGS. 4 and 5, there is an associated increase in theinductive impedance for signals propagating in the odd mode and there isan attendant decrease in the phase velocity. By adjusting the width ofthe aperture 38 or parallel elongated apertures or slots in the groundconductor, the odd mode inductance can be adjusted so that the odd andeven mode phase velocities are substantially equal. By adjusting thewidth of the slot 38 to provide the equal phase velocities, thedirectivity is increased.

In one example of the present invention, a highly directional coupler asillustrated in FIGS. 4 and 5 was operated at a frequency range of from 2to 6 GHz and had the following dimensions: I

Narrow strip-like conductors 33 and 35 were 0.0145 inch wide and were 2to 3 microns thick copper,

Ground conductor 32 was 2 to 3 microns thick copper,

Coupling region 37 was about 7.4 mm. long,

Coupling distance between conductors 33 and 35 at region 37 was 0.0075inch,

Slot 38 was 0.012 to 0.018 inch wide and 13.5 mm. long,

Substrate 31 was 99.5% A1 0 material, dielectric constant of 10.4 andwas 0.025 inch thick.

Referring to FIG. 6, there is illustrated the improved directivity ofthe above-described coupler for signals within the 2 to 6 gigahertzfrequency region applied at terminal 39.

Plot a of F IG. 6 illustrates the coupled power in decibels at thedesired terminal43 relative to the input power applied at terminal 39.Plot b of FIG. 6 illustrates the coupled power in decibels at theundesired terminal 45 relative to the input power at terminal 39. Byreference to plot a, it is shown that signals within the 2 to 6gigahertz frequency region applied at terminal 39 of the above couplerare coupled about 10 db down to terminal 43. By reference to plot 12, itis shown that signals within the 2.to 6 gigahertz frequency regionapplied at terminal 39 of the above coupler are coupled about 30 db downto terminal 45. This means that the directivity, the relative power atterminal 43 relative to terminal 45, was 20 db. This amounts to animprovement on the order of 10 to 15 db for a similar coupler asdescribed above without the aperture in the ground planar conductor.

What is claimed is:

1. In a directional coupler of the type comprising:

a dielectric substrate, a pair of narrow strip-like conductors on onesurface of said dielectric substrate, ,a single ground planar conductoron the surface of said substrate opposite said one surface so as to bespaced directly opposite said strip-like conductors, said pair of narrowstrip-like conductors being closely spaced to each other and to saidground planar conductor so as to provide an electromagnetic couplingregion between said strip-like conductors so that electromagneticsignals applied to said coupler are propagated in odd and even modesalong said pair of conductors at said region, said substrate having adielectric constant of at least six causing a marked difference betweenthe phase velocity of signals propagating in the odd mode and in theeven mode, said ground planar conductor having an elongated aperturewhich extends along said surface opposite said one surface and directlyopposite and parallel to said region for perturbing the odd modesufficiently so that said odd mode phase velocity eqlual said even modePhase velocity. 2. T e combination of 0 arm 1 wherein said elongatedaperture extends at least the full length of the coupling region.

1. In a directional coupler of the type comprising: a dielectricsubstrate, a pair of narrow strip-like conductors on one surface of saiddielectric substrate, a single ground planar conductor on the surface ofsaid substrate opposite said one surface so as to be spaced directlyopposite said striplike conductors, said pair of narrow strip-likeconductors being closely spaced to each other and to said ground planarconductor so as to provide an electromagnetic coupling region betweensaid strip-like conductors so that electromagnetic signals applied tosaid coupler are propagated in odd and even modes along said pair ofconductors at said region, said substrate having a dielectric constantof at least six causing a marked difference between the phase velocityof signals propagating in the odd mode and in the even mode, said groundplanar conductor having an elongated aperture which extends along saidsurface opposite said one surface and directly opposite and parallel tosaid region for perturbing the odd mode sufficiently so that said oddmode phase velocity equal said even mode phase velocity.
 2. Thecombination of claim 1 wherein said elongated aperture extends at leastthe full length of the coupling region.